All fundamental biological processes, including development, immunity, and tumorigenesis, are related to the selective and differential expression of genes in different tissues and cell types. For example, the formation of many malignant tumors has been shown to be associated with the production and/or expression of certain specific cell surface signaling molecules. One of the goals of modern molecular medicine is to find ways to target drugs selectively to reduce or eliminate the drug's off target toxic effects. Delivering drugs to a specific target that is unique to or expressed at higher levels in diseased cells types using targeting moieties such as antibodies, peptides or aptamers has been tried. Attaching these targeting moieties directly to the drug through linkers or to nanoparticles has also been tried.
One such drug targeting system is termed, “antibody drug conjugates” or ADC for short has been studied intensively since 1985 (see, for example, U.S. Patent Publication No. 2009/0220529, incorporated herein by reference). Members of this class of targeted therapeutics are composed of an antibody specific to an antigen, a drug or drugs that act intracellularly, and a linker that connects the antibody to the drug(s). To make ADCs, a wide array of antibodies, linkers, and drugs have been combined and tested in a continuing effort to identify antibodies that specifically target certain cell types and release active drug only upon binding and internalization. Unfortunately, a number of technical difficulties have been encountered with the ADC approach, including the difficulty of finding a means to link the antibody and drug where the linker is stable in the circulatory structure but “unstable” once the ADC has bound to its target or has been internalized into the target cell. Drug release in the circulatory structure before the antibody binds its target or is internalized can lead to undesired toxicity or off target effects. Failure to release the drug after the antibody-target binding or internalization can lead to reduced efficacy. In addition, the linkage must be such that the drug is in an active form when released. Together, these requirements impose considerable design constraints. So it is not surprising that, in all examples to date, ADCs require some sort of separation of the drug from the antibody. In addition, there have been no cases to date where the target for the drug is in close enough proximity to the antibody's target where both the linked drug and antibody act simultaneously. In all cases, acceptable cytotoxicity of an ADC was realized only if some sort of membrane penetration by the drug occurred. In these cases the drug was either released from the conjugate at the target site outside of the cell (see, e.g., U.S. Pat. No. 5,475,092) or after the complete conjugate was internalized.
Another difficulty encountered with the approach relates to how much active drug can be delivered to a target inside the cell by the ADC. Generally, there are only a small number of copies of each different disease-specific antigen binding site at the cell surface, and the number of drug molecules that can be linked to a single antibody without interfering with antigen (target) binding is relatively low (between 5 to 10 per antibody). These two factors in combination have made the ADC approach practical only when very potent (typically very toxic) drugs are used.
Another difficulty encountered with the approach relates to multidrug resistance mechanisms of internalized drugs. For example, cancer cells have the ability to become resistant to multiple different drugs, and share many of the same mechanisms, which include increased efflux of drug (as by P-glycoprotein, multidrug resistance-associated protein, lung resistance-related protein, and breast cancer resistance protein and reproductive cancer resistance protein; enzymatic deactivation (i.e., glutathione conjugation); and decreased permeability (drugs cannot enter the cell). Because efflux is a significant contributor for multidrug resistance in cancer cells, current research is aimed at blocking specific efflux mechanisms.
Yet another drawback with the ADC approach is that the targets have been limited to targets that internalize upon ADC binding. In some cases, even though the target for the ADC exists on the cell surface, internalization does not occur. This makes the ADC approach cell type specific and target specific. Complicating this even further are the cases where the target is expressed and internalization occurs, but the internalization is within compartments where drug antibody dissociation does not occur, leaving the drug ineffective.
Given all these constraints, it is not surprising that Mylotarg [the only ADC approved by the FDA for human therapeutic use (see Hamann, Bioconjug Chem, 13: 40-46, 2002)] was recently removed from the market due to limited efficacy, and no other ADC has been approved to date. Therefore, there continues to be a need for improved ADCs that circumvent these requirements and/or overcome the difficulties and drawbacks of existing methods. The present invention meets that need.